Conventional reciprocating piston internal combustion engines are very complex mechanisms employing a great many parts which are subject to wear and which contribute to losses of efficiency due to friction. Additional energy is lost due to the necessity of reversing the direction of linear motion of the pistons and the conversion of the linear motion to rotary motion. The reciprocating motion of the pistons also creates vibrations which contributes to the failure of components of such engines and structures to which they are attached. In spite of the problems and inefficiencies of reciprocating piston engines, such engines have been very successful and form the majority of prime movers for ground transportation and many other uses.
Throughout the twentieth century, attempts have been made to develop entirely rotary internal combustion engines to overcome the inherent problems of linear piston engines. Few attempts have been successful. While many such engines appeared to have theoretical potential, it has been difficult or impossible to construct practical working models of such engines. The only well-known rotary engines which have achieved practical success are gas turbine engines and the Wankel rotary internal combustion engine.
In general, gas turbine engines have been applied most successfully to aircraft propulsion as fairly large engines. While a few small engines have been built and tested in passenger car size ground vehicles, the disadvantages of small gas turbine engines have outweighed the advantages. Maintenance of such engines is more expensive than for a comparable size piston engine. Additionally, the engine power relative to engine speed range is narrower for gas turbine engines whereby ground vehicles would require more complex and thus more expensive transmissions than are needed for piston engine driven cars.
The Wankel rotary engine, in contrast, has been applied fairly successfully to smaller ground vehicles such as automobiles and motorcycles, although not on a great scale and not without problems. Initially, there was a problem with excessive wear of rotor apex seals. Such problems have been overcome for the most part by the use of improved materials and manufacturing techniques. While the major component of motion of the Wankel rotor is, as its name implies, rotary, there is additionally a reciprocating component since the housing cavity in axial cross section is elongated rather than circularly cylindrical. The reciprocating component of rotor motion is a consequence of the geometry of the rotor and housing, the manner of gearing the rotor to the engine shaft, and the manner in which the fuel-air mixture is compressed and exhausted after combustion. This causes some loss of efficiency since energy must be expended in accelerating and decelerating the rotor through its linear components of motion.